Defrosting device, in particular for refrigeration systems

ABSTRACT

A device for defrosting the finned cooling assembly of a refrigeration system having an ellipsoid transparent polymer material housing a light source. The light source is positioned substantially in correspondence with one of the focal points of the ellipsoid. A sensor for the light beam radiated by the light source and reflected by an inner surface of the ellipsoid recognize variations in the luminous intensity of the beam caused by the formation of frost along the path of the light beam. A plurality of finned cooling assemblies are activated by the signal emitted by the sensor.

This invention relates to a defrosting device, in particular forrefrigeration systems.

In refrigeration units, the finned assembly is defrosted by the use ofheating elements in contact with the assembly itself. Said heatingelements are in the form of electrical resistance elements, which arepowered at predetermined time intervals for a constant period. Ifexcessive heating occurs, a safety thermostat ensures that the itemscontained in the refrigeration unit remain preserved, by deactivatingthe heating elements.

This known defrosting system has the drawback of non-optimum defrostingof the finned because the powering of the heating elements is notcorrelated with the effective presence of frost. This can result in:

activation of the heating elements even if frost is not present on thefinned assembly, with consequent energy consumption and increase in thetemperature of the environment, resulting in damage to the food,

inefficient heat transfer by the refrigeration assembly due to thepresence of frost because of delay in operation of the heating elements.

This drawback is eliminated according to the invention by a device fordefrosting the finned cooling assembly of a refrigeration system asdescribed in claim 1.

A preferred embodiment of the present invention and one modificationthereof are described in detail hereinafter by way of non-limitingexample with reference to the accompanying drawings, in which:

FIG. 1 is a schematic view of a first embodiment of the device of theinvention,

FIG. 2 shows it mounted on the finned assembly, and

FIG. 3 shows a modification thereof.

As can be seen from the figures the device of the invention consists ofan ellipsoid portion 2, of eccentricity between 0.4 and 0.75,constructed of transparent polymer (for example polypentenes, PMMA,water-saturated transparent polyamides, etc.) having a refractive indexto air of about 1.49.

Within the ellipsoid 2 there is provided a seat 4 housing a lightemitting diode 6 (preferably a 940 nm IRED) positioned preferably inproximity to a focal point of the ellipsoid. The diode seat 4 is closedby a plug 8 of black plastic. Said ellipsoid portion is prolonged in theform of two concentric tubes 10, 12, in one of which, namely in theinner tube 10, there is housed a photodetector 14 connected to a controlunit for an electrical resistance element 16 mounted on a bracket 18facing the finned assembly 17 of the refrigeration unit 19.

In particular, said photodetector is positioned substantially incorrespondence with the other focal point of the ellipsoid.

In the annular compartment bounded by the two tubes 10, 12 there ishoused a copper tube 20 having that end surface facing the ellipsoidsubstantially inclined to the tube axis so as to define with theellipsoid an annular aperture 22 having two walls formed by portions ofthe tubes 10, 12, one wall formed by the copper tube 20, and one wall 24formed by the body of the ellipsoid itself.

That end of the copper tube external to the ellipsoid is rigid with thebracket 18, so as to achieve good thermal contact.

The device of the invention operates in the following manner: when frostis absent, because of the geometrical characteristics of the ellipsoidthe light rays leaving the diode 6 strike a region of the ellipsoidsurface 5 with an angle of incidence (>43°) which is greater than thelimiting angle corresponding to the refractive index of the ellipsoid toair. Consequently the rays r are not refracted, but instead arereflected within the ellipsoid itself, to then emerge therefrom throughthe annular band 24 which lies substantially perpendicular to the rays.The light rays then interfere with the inner annular portion of the tube10, to undergo double refraction and be conveyed onto the photodetector12, which consequently receives all the light rays emitted by the diodeonto the surface S.

In this configuration, in which the intensity of the radiation receivedby the photodetector is a maximum, the resistance element 16 is notpowered.

At that moment in which, during the operation of the refrigeration unit,frost microcrystals form on the outer surface of the ellipsoid, therefractive index of the ellipsoid to said microcystals covering itvaries by virtue of the presence of these covering microcystals. As thisrefractive index decreases, with consequent increase in the limitingangle corresponding to the new refractive index, the rays radiated bythe diode and which strike this region are no longer reflected, butinstead are refracted with consequent isotropic light diffusion to theoutside.

This situation consequently results in a decrease in the luminousintensity of the rays sensed by the photodetector. Moreover the presenceof frost on the annular surface 24 causes a maked defocusing of thebeam, contributing to accelerating the extinguishing of the light beamreceived by the photodetector. Consequently the photodetector feeds adifferent signal to the control unit, which compares it with the basicsignal previously received when all the rays struck the photodetector,the control unit causing the resistance element 16 to operate when theratio of the attenuated signal to the basic signal reaches the order of50%.

The activation of the resistance element 16 results both in thedefrosting of the finned assembly 17 and, by virtue of the heatconducted through the bracket 18 and the annular tube 10, the defrostingof the outer surface of the ellipsoid body, which consequently returnsto its initial state in which the photodetector 12 receives virtuallyall the rays transmitted by the diode. The control unit consequentlyreceives a signal substantially similar to the basic signal, toconsequently deactivate the resistance element.

In the embodiment shown in FIG. 3, the device of the invention consistssubstantially of a truncated portion of ellipsoid 26, also oftransparent polymer material but with its outer surface 5 metalized.Along the major axis of the portion there is provided a seat 28 housinga photodetector 30, and a light source 32 in the form of an LEDpositioned substantially in correspondence with the focal point of theellipsoid.

A black mask 34 is interposed between the light source 32 and thephotodetector 30 so that the rays emitted by this source cannot directlystrike the photodetector.

External to the ellipsoid, in correspondence with its substantiallycircular surface 36, there is provided a metal needle 38 with itsburnished mirror-like tip positioned to correspond with the positionwhich would have corresponded with the other focal point of theellipsoid if this had not been truncated. The other end of the needle isin thermal contact with the finned assembly 17.

The circular surface 38 and the end surface of the seat 28 are curvedand shaped to form a micro-lens 42.

This embodiment of the device of the invention operates in the followingmanner:

under normal conditions the light rays r radiated by the LED 30 arereflected by the metallized surface of the ellipsoid and fed to the tipof the needle. When in this configuration the photodetector receives noradiation and consequently feeds no signal to the control unit foractivating the resistance element. In the same manner the burnished tip39 generates to radiation towards the photodetector, even if itinterferes with the beam. At that moment in which, as result of theoperation of the refrigeration machine, a layer of frost forms on thefinned assembly, the thermal connection between the metal needle 38 andthe finned assembly 17 creates the same frost conditions on the tip 39as on the finned assembly. The increase in the frost thickness on theneedle causes its frost crystals to interfere with the light beanradiated by the LED and reflected by the surface S of the ellipsoid,with the formation of an isotropic light source with conjugate image bythe LED primary source, which via the micro-lens 42 is fed to thephotodetector 30, which feeds a signal to the control unit causing it toactivate the resistance element in order to defrost the finned assembly17.

In this case the defrosting of the finned assembly also causesdefrosting of the needle tip. The result is the restoration of normalconditions with no radiation received by the photodetector, which feedsa signal to the control unit to deactivate the resistance element.

From the aforegoing it is apparent that the device presents numerousadvantages, and in particular:

it provides rapid and effective defrosting of the finned assembly as theresistance element operates only if frost is present,

energy costs are reduced as it operates only when frost is present,

accurate control is obtained on the basis of the thickness of the frostpresent on the distributor.

What is claimed is:
 1. A device for defrosting the finned coolingassembly (17) of a refrigeration system, characterised by comprising: anellipsoid (2,26) of transparent polymer material housing a light source(6,32) positioned substantially in correspondences with one of the focalpoints, and a sensor (12,30) for the light beam radiated by said sourceand reflected by the inner surface of the ellipsoid, a plurality ofheating and defrosting elements (18) for the finned cooling assembly(17), which are activated by a signal emitted by the sensor (12,30) whenthis senses a variation in the luminous intensity of the beam, saidvariation in luminous intensity of the beam being caused by theformation of frost along the path of the light beam.
 2. A device asclaimed in claim 1, characterised in that said ellipsoid (2) has arefractive index to air and an eccentricity such that if frost is absentthe rays emitted by the light source (6) are reflected directly onto thephotodetector (12), whereas if frost is present said rays are refractedso that the photodetector senses a variation in luminous intensity.
 3. Adevice as claimed in claim 2, characterised in that said ellipsoid has arefractive index of 1.49 and an eccentricity of between 0.4 and 0.75. 4.A device as claimed in claim 2, characterised in that said ellipsoid isprovided with a conducting tube (20) thermally connected to the heatingelements (18), so that activation of said elements (18) also causes thefrost on the ellipsoid surface to melt.
 5. A device as claimed in claim1, characterised in that said ellipsoid (2) has a metallized outersurface and comprises, in correspondence with the other focal point, ablack body which following the formation of frost interferes with therays radiated by light source, to feed them to the photodetector via anoptical system (42).
 6. A device as claimed in claim 5, characterised inthat said black body consists of a needle with its tip positioned incorrespondence with the focal point and with its other end in contactwith the finned assembly.